1. Field of the Invention
The present invention generally relates to density regulating apparatus for developing liquid for use in image forming apparatuses such as a copying machine, a facsimile machine, a printer and the like. The present invention relates more particularly to a density regulating apparatus for regulating a density (solid content density) of developing liquid so that a predetermined density level is attained, without supplementing developing liquid, and also to an image forming apparatus provided with such a density regulating apparatus.
2. Description of the Related Art
Various types of image forming apparatuses are known in which developing liquid formed as dispersion of toner particles (solid content) in carrier liquid is used, in order to develop a latent image and form a toner image on a surface of a recording member.
FIG. 1 is a front view showing a schematic construction of an image forming apparatus in which high-density developing liquid is used. Since the high-density developing liquid has a significantly high toner particle density, the developing liquid as a whole has a high viscosity (100-10000 mPa.cndot.s) even when the carrier liquid is low in viscosity. Generally, in an image forming apparatus in which high-density developing liquid is used, a pre-wet process whereby a pre-wet film is formed on a photosensitive object (a latent image carrying object) by applying a pre-wet liquid on the photosensitive object, in order to prevent the background of the image from being soiled.
Referring to FIG. 1, high-density developing liquid 20 in a developing liquid tank 21 is supplied to the surface of a developing belt 2 via the surfaces of a series of developing liquid applying rollers 22a, 22b and 22c. In this process, the developing liquid is transformed into a thin film so that a developing liquid film having a regular thickness is formed on the developing belt 2.
Transparent pre-wet liquid is applied by the pre-wet roller 6 to the surface of a photosensitive drum 1. An electrostatic latent image is formed on the surface of the photosensitive drum 1 uniformly charged by a charging roller 7. The latent image is developed by the developing liquid film on the developing belt 2. The developed image on the photosensitive drum is transferred to paper 8 (recording member) fed to a position opposite to the photosensitive drum 1. The paper 8 having the image transferred thereto passes between the photosensitive drum 1 and a transfer roller 3 and reaches a fixing unit (not shown) so that the image is fixed. The paper 8 is then ejected outside the apparatus.
When the image has been transferred to the paper 8, the photosensitive drum 1 has toner remaining thereon raked off by a cleaning blade 4. A discharging lamp 5 initializes the photosensitive drum 1 by removing the charge remaining thereon. The developing liquid that remains on the surface of the developing belt 2 past an area where a developing process occurs is raked off by a collecting blade 23 so that the developing belt 2 is initialized. The developing liquid past a development area is drawn in by a pump 24 so as to be returned to developing liquid tank 21 via a developing liquid drain passage 25. The developing liquid returned to the developing liquid tank 21 is re-used.
The toner in the developing liquid is partially consumed as a result of the developing process. In addition, if the pre-wet liquid has the same content as the carrier liquid, the developing liquid and the pre-wet liquid are mixed with each other so that the developing liquid past the development area has the pre-wet liquid mixed therein. Accordingly, the toner density of the developing liquid past the development area is lower than the toner density required in the developing process (hereinafter, the toner density required in the developing process will be referred to as a predetermined toner density). Accordingly, if the developing liquid past the developing area is returned as it is to the developing liquid tank 21 for re-use, the density of the image formed on the paper 8 is decreased.
According to one conventional approach, decrease in the density of the image is prevented by providing a density sensor 26 for detecting a toner density of the developing liquid 20 in the developing liquid tank 21, and a high-density developing liquid tank 27 for containing developing liquid having a higher toner density than the predetermined toner density. The density sensor 26 is designed to detect a decrease in the toner density of the developing liquid 20 in the developing liquid tank 21. By supplementing the high-density developing liquid of the high-density developing liquid tank 27 to the developing liquid tank 21 appropriately, the toner density of the developing liquid is maintained at a constant level.
Referring to FIG. 1, also connected to the developing liquid tank 21 so as to be located side-by-side with respect the high-density developing liquid tank 27 is a carrier liquid tank 28 for retaining the carrier liquid. For example, when dust or the like is mixed in the developing liquid 20 in the developing liquid tank 21, the developing liquid is discarded so that the developing liquid tank 21 is supplied with the high-density developing liquid from the high-density developing liquid tank 27 and with the carrier liquid from the carrier liquid tank 28, with a mixture ratio controlled to achieve the predetermined toner density.
FIG. 2 is a front view showing a schematic construction of an image forming apparatus in which a low-density developing liquid having a viscosity of 100 mPa.cndot.s or below is used. Like the image forming apparatus of FIG. 1 in which the high-density developing liquid is used, the image forming apparatus of FIG. 2 is constructed such that the developing liquid on the developing roller 2 past the development area and having a reduced toner density as a result of the developing process is returned to the developing tank 21. When the density sensor 26 detects a decrease in the toner density of the developing liquid 20 in the developing tank 21, the high-density developing liquid tank 27 supplies the high-density developing liquid to the developing tank 21 so that the toner density is maintained at a constant level. A squeeze roller 9 may be used to remove the leftover carrier liquid (rarefied developing liquid) past the development area on the surface of the photosensitive drum 1.
However, the image forming apparatus in which high-density developing liquid is used and the image-forming apparatus in which low-density developing liquid is used as described above have a drawback in that the volume of the developing liquid in the developing tank 21 continues to increase as a result of the high-density developing liquid being supplied to the developing liquid tank 21 in order to prevent the toner density of the developing liquid in the developing liquid tank 21 from being decreased, causing an overflow of the developing liquid in the developing liquid tank 21.
The aforementioned drawback is particularly serious in an image forming apparatuses in which low-density developing liquid is used and in which leftover carrier liquid (rarefied developing liquid) past the development area on the surface of the photosensitive drum 1 is removed by the squeeze roller 9 so as to be returned to the developing liquid tank 21. Since a large quantity of high-density developing liquid should be supplemented to the developing liquid tank 21 in this construction, an overflow of the developing liquid is likely to occur at a relatively short interval.
Once conceivable approach to eliminate the above-described drawback is to increase the toner density of the developing liquid in the high-density developing liquid tank 27 so that the quantity of the developing liquid supplemented to the developing liquid tank 21 is reduced, while ensuring that a proper developing performance is not lost. However, while such an approach is useful to extend a period of time required for an overflow to occur, it cannot prevent the volume of the developing liquid in the developing tank 21 from being continually increased.